Inverter drive systems that drive an electrical rotating machine such as an electrical motor by means of an inverter have been developed and become widespread. In such an inverter drive system, the inverter converts a DC voltage into a pulse voltage by switching operation and supplies the pulse voltage to a rotating electrical machine through cables. The rotating electrical machine is driven by the supplied pulse voltage.
Conventionally, in a high voltage rotating electrical machine, an electric field relaxation system is often provided on a coil surface near a stator core end portion for the purpose of preventing occurrence of partial discharge especially in a portion near the core end portion of the stator coil or heat generation therefrom. The electric field relaxation system is a combination of a low resistance layer derived from a stator core slot and an electric field relaxation layer formed partially on the low resistance layer.
In the inverter drive system, reflected waves are generated due to impedance mismatch between the inverter, the cables, and the rotating electrical machine. When the generated reflected waves are superimposed on the pulse voltage, high voltage noise, so-called inverter surge may occur between the cables and the rotating electrical machine, especially, at connecting portions between the cables and the rotating electrical machine.
When the pulse voltage including the inverter surge (hereinafter, referred to as “inverter pulse voltage”) repeatedly occurs, the partial discharge or heat generation, which are not caused during operating time by commercial frequency, is caused at the stator coil of the core end portion (hereinafter, referred to as “stator coil end”). Further, the partial discharge or heat generation that may cause deterioration in reliability-related characteristics occurs on the electric field relaxation system. As a result, reliability of the stator coil may be significantly lowered.
Occurrence of the partial discharge or heat generation depends on the gradient of a surface potential of the potential relaxation system (see Non-patent Document 1). Thus, there has been a strong need for a technology that accurately measures the surface potential of the electric field relaxation system in which the inverter pulse voltage is generated.